Arc lamp construction



R. E. PAQUETTE ARC LAMP CONSTRUCTION June 30, 1970 Filed Oct. 24, 1967 Fig 3B INVENTOR RAYMOND E. PAQUETTE lwLML A ORNE Y 2 Sheets-Sheet 1 June 30, 1970 R. E. PAQUETTE 3,518,480

ARC LAMP CONSTRUCTION Filed OCT.. 24, 1967 3 Sheets-Sheet i INVENTOR.

RAYMOND E. PAQUETTE BY LNLML i ATTORNEY United States Patent O 3,513,480 ARC LAR/ll CONSTRUCTION Raymond E. Paquette, Saratoga, Calif., assignor to Gevaert-Agfa NN., Mortsel, Belgium, a Belgian company Filed Oct. 24, 1967, Ser. No. 677,565 lut. Cl. HOlj I 96 U.S. Cl. 313-288 1S Claims ABSTRACT F THE DISCLOSURE An arc lamp construction in which the lamp envelope portion surrounding the main body portions of the arc electrodes is spaced therefrom to denne a cylindrical gap and in which a packing member is disposed in the cylindrical gap to support the electrodes. The packing member is elastic, or radially deformable, to prevent cracking of the envelope as the gap width changes with temperature, and is thermally conductive to allow the transfer of electrode heat across the gap to the envelope.

BACKGROUND OF THE INVENTION Field of the invention The invention relates to arc lamps, and, more particularly, to an arc lamp construction that is capable of withstanding severe shock and vibration by the provision of an electrode support in the nature of a soft packing that also aids in maintaining precision alignment of the electrodes and improves the heat transfer from the electrodes to the envelope, and thereby lengthens the useful operating life.

For certain arc lamp applications the arc lamp is exposed to an environment which makes extremely high demands on the mechanical and electrical integrity of the lamp. For example, in missile and rocket applications, an arc lamp has to withstand severe shock and vibration, and consequently requires very firm anchoring of its electrodes to prevent electrodes from shaking loose and to maintain a constant arc gap. Further, since weight and space are always problems in missile and rockets, a light and small arc lamp is always desired. Since the required useful lifetimes are usually short, such arc lamps can be run above their normal rating for short times thereby allowing a much lighter and smaller arc lamp to be used.

The major problem encountered in arc lamp constructions which have good survival capabilities to vibration and shock, and which may be overdriven to provide a minimum required amount of illumination far above its normal rating, is the fact that the refractory electrode material has a coefficient of expansion which is about eight times greater than the radiation transparent envelope material. Accordingly, to securely support the electrodes within the envelope requires the envelope to be tightly or hard shrunk upon the electrode body. However, if this is done when the electrode is cold, the subsequent heating of the electrodes during arc operation will fracture the envelope because of the unequal rate of expansion. If the envelope is hard shrunk upon the electrode to provide a tight fit at normal operating temperatures, then the electrodes will be loose prior to reaching their normal operating temperature, and will be subject to destruction when exposed to shock and vibration. Additionally, it is almost impossible to control manufacturing tolerances so closely that a good tight fit between the envelope and the electrode at a usually ever increasing operating temperature can be accomplished. This is particularly true since heat dissipation from the electrode to the envelope is best when the contact is good.

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Description of the prior art One type of arc lamp which is particularly suited for missile type applications is the self starting short arc lamp which utilizes a bridgewire across the arc gap which is exploded to start the lamp. This lamp is described in US. Pat. Nos. 3,256,459 and 3,274,427 and in copending patent application Ser. No. 554,933, filed May 19, 1966, now Pat. No. 3,377,497, entitled Self Starting Arc Lamp and assigned to a common assignee.

Even though a bridgewire short arc lamp performs satisfactorily, its survival capability, arc stability and uesful life duration, when exposed to severe shock, vibration and power overloads, has been found Wanting in some respects. For example, when the lamp is constructed to withstand shock and vibration when cold (no arc), there is always danger of envelope cracking when the lamp is operating because of electrode expansion. When the lamp is constructed to withstand shock and vibration when hot, shock and vibration damage prior to reaching operating temperatures are common, and, additionally, the heat dissipation is poor, thus shortening useful life.

Even though this invention will be explained with particular reference to self-starting short arc lamps, it is to be understood that it is equally applicable to ordinary arc lamps, whether short or long arc, and whether started with a firing pulse or a bridgewire. The present invention can be employed Wherever there are problems of shock, vibration, electrode alignment or heat transfer in lamps in which electrodes are usually cantilevered into the bulb from the electrode foot.

SUMMARY OF THE INVENTION It is therefore a primary object of the invention to pro- Vide an arc lamp having improved electrode supports and improved electrode heat dissipation for greater shock and vibration survival capacity and longer useful life of the lamp.

It is another object of the invention to provide an improved electrode support for facilitating and maintaining precision alignment of the electrodes in an arc lamp.

It is a further object of the invention to provide structure between a portion of an electrode and the inner periphery of the lamp envelope for improving the heat transfer characteristics of an arc lamp thereby providing longer life for the lamp.

It is still another object of the invention to provide a flexible or resilient or elastic or radially compressible and expandable or soft electrode support packing for an arc lamp resulting in an improved shock and vibration survival capability, a better alignment of the electrodes, and an improved heat transfer from the electrode to the envelope.

It is another object of the invention to provide a resilient electrode support for facilitating manufacture of the arc lamp to provent breakage thereof.

It is a further object of the invention to provide a method of manufacture of an arc lamp wherein flexible resilient electrode support packing is utilized to compensate for the different coeflicients of expansions of the electrodes and the envelope.

Briey, one embodiment of an arc lamp of this invention accomplishes the stated objects by utilizing a helical coil of molybdenum wire wound around a portion of the exterior surface of each electrode for approximately eight turns. The wire coil is in the form of a helix or a spiral and has a formed interior diameter which is slightly smaller than the diameter of the electrode so that the coil is, in effect, sprung over the electrode for securement thereto. In the alternative, and for greater securement,

the wire coil may also be fixed to the electrode at a single point by a spot weld or the like.

The interior surface of the envelope is ridged or grooved round the individual coils of Wire to prevent dislodgement in a dierction along the axis of the lamp. When the lamp is cold, there is a very small gap between the coil and either the external surface of the electrode or the internal surface of the envelope to allow for expansion. When the coil of wire becomes heated during operation of the arc lamp, the heating causes the coil of wire to expand as by winding along the longitudinal eX- tent of the electrode. With the coil of wire being flexible or resilient, and having the ability to wind and unwind in the peipheral space between the exterior surface of the electrode and the interior surface of the envelope, the electrode is thus flexibly and resiliently supported within the envelope both when hot and cold, and the likelihood of breakage of the envelope by heating or from vibration and shock is -greatly reduced. Also, the juxtaposition of the coil relative to the electrode facilitates alignment of the electrodes both during assembly of the arc lamp and while the lamp is in use. Further, the coil of wire surrounding the electrodes, being metallic and in contact with both the electrode and the envelope, improves the heat transfer from the electrode during arcing of the lamp and thus prolongs the life of the arc lamp.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a View in elevation of self-starting short arc lamp constructed in accordance with this invention;

FIG. 2 is an enlarged view, partly in elevation and partly in cross section, of a portion of the arc lamp of FIG. l illustrating the electrode with a coil wire packing member;

FIG. 3A is a view, partly in elevation and partly in cross section, of a modification of the invention;

FIG. 3B is a view taken along lines 3B-3B of FIG. 3A;

FIG. 4A is a View, partly in elevation and partly in cross section, of a further modification of the invention;

FIG. 4B is a view taken along lines 4B-4B of FIG. 4A;

FIG. 5A is a view, partly in elevation and partly in cross section, of another modification of the invention;

FIG. 5B is a View taken along lines SB-SB of FIG. 5A;

FIG. 6A is a view, partly in elevation and partly in cross section, of still another modification of the invention; and

FIG. 6B is a view taken along lines 6B-6B of FIG. 6A.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, in which like reference numerals designate like parts, and particularly to FIGS. 1 and 2, a short arc bridgewire lamp 10 is shown constructed in accordance with the invention. Lamp 10 includes a lamp envelope 12 usually made of quartz for well-known reasons which has typically a thermal coeiicient of linear expansion of about 4.9 10*5 per C. Mounted Within lamp envelope 12 are a pair of electrodes 13 and 14 which respectively form the anode and cathode of lamp 10, and which are made of a refractory metal such as swaged tungsten which typically has a thermal coecient of linear expansion of about 42x10*5 per C. Electrodes 13 and 14, respectively, include a main body portion 15 and 16, foot portions 20 and 21, and yarc end portions 35 and 36. Electrodes 13 and 14 may be identical, as shown in the drawings, so that either may vbe selected as the cathode thereby making the lamp nonpolarized. Of course, if desired, one electrode may be constructed of `a larger diameter than the other to form the hotter anode as is usually done in arc lamps. However, the fact that the lamp may be made nonpolarized is an advantage in that savings are made in manufacturing cost, and the user can connect the voltage supply to the lamp without regard to polarity. If lamp 10 is of the self starting type, a bridgewire 50 is connected across the electrode gap.

Surrounding body portions 15 and 16 of the electrodes are a pair of wires 17 and 13 which, in the preferred embodiment, are made of molybdenum having a thermal coefficient of linear expansions of about 50X l0.-5 per C. Molybdenum wires 17 and 1S are formed into halical coils of approximately six to eight turns, and the coils, prior to being placed on the electrodes, have an internal diameter `which is slightly smaller that the diamether of the electrode body portion. The coils are, in effect, sprung over the electrode body portion to hold them in place. For additional asurance, the wire coils may also be secured to the electrodes at a single location, as at 19 in FIG. 2, to allow the remainder of the coil to wind and unwind over the exterior surface of the electrode body portions, as will be described hereinafter.

Electrodes 13 and 14 are primarily supported, in cantilever fashion, within envelope 12 by shrinking the portion of envelope 12 coextensive with electrode feet 20 and 21 about the electrode feet. Wire coils 17 and 18, as will become clearer from the ensuing description, supplement the cantilever support and further secure the electrodes against longitudinal and lateral displaceemnt when the envelope portion coextensive with the wire coils is shrunk about the wire coils.

This shrinking is accomplised as an additional step in the manufacture of the lamp after the envelope has been shrunk around electrode feet 2t) and 21 and sealed over the ribbons. More particularly, the arc lamp, which is under negative pressure for well-known manufacturing reasons, is heated uniformly to a temperature which corresponds to its operating temperature which is between 1400 and l500 C. This causes the electrodes, the wire coils, and the envelope to expand in accordance lwith their respective thermal coeicients of linear expansion and assure their sizes at operating conditions.

Thereafter, the portion of envelope 12 coextensive with wire coils 17 and 18 is heated to about 1650 C. which is close to the softening point of the quartz. Because of the negative pressure inside envelope 12, the heated portion, now softened, moves inwardly under the negative pressure until it comes into Contact with wire coils 17 and 18. This corresponds to a hard shrink of the envelope upon the wire coils. The degree of penetration of envelope 20 into the spaces between the coils is carefully controlled by the flame so that good sized corrugations or grooves 22 are formed in the interior surface of the envelope. After a desirable degree of penetration is obtained, the arc lamp envelope is allowed to cool which causes electrodes 13 and 14, coils 17 and 18, and envelope 12 to contract. Since the thermal coeicient of linear expansion of the quartz is approximately ten times less than that of the tungsten electrode, or the molybdenum wire, it is readily seen that coils 17 and 18 will shrink away from the quartz envelope and thereby change the hard shrink support to a soft support. For a wire coil, which is typically constructed of 0.015 inch diameter wire, and which surrounds an electrode having a diameter of about oneeighth of an inch, it has been found that the shrink-away is about 0.001 inch.

It is to be noted at this point that whenever the arc lamp is at its operating temperature, a hard shrink condition exists between the envelope, the wire coils and the electrodes. This condition is not dangerous to the integrity of the envelope since the envelope was shrunk on the coil-electrode combination at this temperature and the forces, due to expansion of the electrodes, are not in excess of those which caused the envelope to shrink about the electrode in the first place during manufacture. Accordingly, during operating condition, there is firm lateral and longitudinal support of the electrodes and good thermal contact from the electrodes to the envelope which is due to the hard shrunk condition.

When the arc lamp is cold, wire coils 17 and 18 becorne elastic springs which resiliently support the electrodes within the envelope. As wire coils 17 and 18 contract, during cooling, they become shorter and wind up upon the electrode body using grooves 22 as guides which they threadably engage. This is best seen in FIG. 2 where wire end 19 may be secured to the electrode, and wire end 19a is movable through a distance D during winding and unwinding. When the lamp is cold, it can be said that the electrodes are flexibly or resiliently supported by the coils of wire within the envelope. Also, it can be seen that the coils of wires 17 and 18 help maintain the electrodes 13 and 14 in alignment both during assembly and during use and, most importantly, if the coils of wires are firmly secured to their respective electrodes, then grooves 22 secure the electrodes against axial displacement,

Foot of electrode 13 is connected to a pair of conductive ribbons 25 and 26 by spot welds to molybdenum wire (typically .006 inch) tabs 27 and 28, one on each side of electrode foot 20 as shown in FIG. l. Ribbon 25 is typically made of molybdenum having a width of oneeighth of an inch and a thickness of 0.7 mil. The other end of ribbon 25 is similarly connected to a terminal lug 30 to which a flexible lead 31 is affixed. The advantage of having a pair of ribbons 25 and 26 at the electrode foot is that a layer of metal ribbon is thus superimposed between the electrode foot and the quartz envelope. Such superimposed layer of metal ribbon helps prevent breakage as the quartz envelope is shrunk in relation to the interior lamp assembly just described.

A similar metal ribbon construction is connected to the other electrode and to a terminal lug 32 to which flexible lead 33 is attached. The lamp construction is generally known as a ribbon seal lamp since the seal between the lamp interior and the outside is formed around the ribbons. It is to be understood that the present invention is equally applicable to rod seal lamps in which the ribbons are replaced by rods and where a seal is formed around the rods.

Thus, the components of a short arc bridgewire llamp have been described wherein a exible resilient electrode support feature has been described in conjunction with coils of wire `17 and 18 surrounding body portions 15 and 16 of electrodes 13 and 14.

Before describing a number of modifications of the electrode support shown in FIGS. 1 and 2, it may be helpful to delineate the general support requirements which are met by the wire coils. As a consequence of the cantilever support of the electrodes in the lamp envelope, the basic survival capability of the arc lamp to severe shock and vibration is low. Shrinking the envelope upon the body portion of the electrode when the same is below operating temperature is not possible because the electrode has a much greater thermal coefficient of linear expansion than the envelope and would, therefore, crack the envelope when expanding during arc operation. Shrinking the envelope directly upon the body portion of the electrode when the same is at operating temperature is likewise not the answer because, when the lamp is cold, the electrode is not securely supported against lateral and longitudinal displacement. Accordingly, what is required is a soft support when the lamp is cold and a hard support when the lamp is hot, the hard support corresponding to shrinking the envelope upon the electrode.

In accordance with this invention, a cylindrical space 51 is provided between the exterior surface of the main body of the electrode and the corresponding interior surface of the envelope. Disposed in this space is a packing with certain properties. The packing substantially fills the cylindrical space to thereby support the electrode within the envelope at any electrode temperature. The

packing provides a thermal path across the space to allow for the dissipation of heat. Finally, the packing not only provides lateral support by filling the space, but may also provide longitudinal support by afxing the packing to the electrode and shaping the envelope as is obtained by the coils of wire described in connection with FIGS. 1 and 2 which are secured to the electrodes and threaded in grooves 22.

The above characteristics of the packing are provided by a metallic packing which is radially compressible and expandable to accommodate itself to the variation of the radial gap dimension due to the enormously different thermal coeiicient of linear expansion. The coil springs meet these requirements since they are elastic and in contact with the electrode and the envelope when cold or hot. When hot, the clearance disappears and good support and thermal conductivity by a hard shrunk fit is assured. With these basic concepts in mind, it will now be understood that a large number of modifications of the packing are possible without departing from the spirit of the present invention.

FIGS. 3A and 3B disclose a modification of the invention wherein the packing member about which the envelope 12 is shrunk takes the form of a metallic refractory ribbon spring 52 which is wound several times around ybody portion 1S of electrode 13. The interior end 53 of the metallic ribbon spring 52 is secured to the electrode as by spot welding or the like, while the other end 54 is allowed to remain free. Thus, the coil of metallic ribbon 52 is free to wind or unwind relative to the body portion of the electrode and within the deformed portion of the envelope shrunk thereabout as the ribbon expands and contracts as a result of ambient conditions. The Imetallic ribbon 52 thereby resiliently supports the electrode 13 to withstand vibration and shock by the cushioning action of the wrapped ribbon and the freedom of expansion and contraction of the ribbon 52. Furthermore, the ribbon engages the edge 49 of the annular groove formed in the envelope 12 as it was shrunk thereabout to provide the desired longitudinal integrity. The metallic ribbon 52 wound around the body portion 15 also affords the advantages of improved heat transfer from the electrode to the envelope, and aids in maintaining the electrodes in alignment.

FIGS. 4A and 4B disclose a further modification of the invention wherein the packing member takes the form of a corrugated cylinder 60 having ridges 61 in contact with the interior surface of envelope 12 and trough 62 in contact with body portion 15 of electrode 13. To secure packing member `60 to electrode 13, portion 15 may be threaded or a portion such as 62 may be spot welded to electrode 13. Likewise, for longitudinal integrity, envelope 12 may be shrunk over corrugated cylin der 60 in a manner similar to that described in connection with shrinking the envelopes over the coils of wire. Elasticity, or radial compressibility, provides the necessary adjustment for the variation in the gap distance, and thereby supports electrode 13 either in the hot or cold condition. Further, good thermal transfer characteristics are associated with a corrugated cylinder 60.

FIGS. 5A and 5B show a still further modification of the invention wherein the packing member takes the form of a lengthwise corrugated cylinder 70 having crest at 71 for engaging the interior surface of envelope 12 and valley 72 for engaging body portion 15 of electrode 13. FIGS. 6A and 6B disclose yet another modification of the invention wherein the packing member takes the form of a cylinder which includes a number of outwardly projecting spaces 81 for engaging the interior surface of envelope 12. The interior surface 82 of cylinder 80 may be shrunk over body portion 15 of electrode 13.

In the embodiments shown in FIGS. 3A, 4A, 5A and 6A, longitudinal support may be provided by shrinking envelope 12 over the points of contact with the packing member. Lateral stability is usually provided by the twopoint contact provided by different surfaces of the packing member with the interior envelope surface and exterior electrode surface.

Thus, an arc lamp having flexible resilient electrode supports has been described wherein damage from vibration and shock is reduced, heat transfer is improved, and alignment of the electrodes is facilitated and maintained, thereby improving its overall operation of the arc lamp and its longevity.

While the above detailed description has shown, described and pointed out the fundamental novel features of the invention as applied to various embodiments, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated may be made by those skilled in the art, without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

What is claimed is:

1. In an arc lamp construction in which a pair of electrodes are end supported in spaced apart relationship within and by a sealed envelope which has a thermal coeiiicient of linear expansion several times smaller than the electrodes, the improvement in supporting the electrodes against axial displacement comprising:

a peripheral gap between the external surface of unsupported portions of said electrodes and the internal surface of said envelope coextensive with said electrode portions; and

radially compressible packing members aiiixed to said external surface and disposed within said gaps, said envelope being shrunk about said packing members so as to form grooves in said envelope for receiv ing said packing members, said packing members engaging the edges of said grooves to support said electrodes securely against longitudinal as well as lateral displacement Within said envelope.

2. In an arc lamp construction in accordance with claim 1 in which said packing members are comprised of a refractory metal and form conductive heat paths across said peripheral gaps.

3. In an arc lamp construction in accordance with claim 2 in which each packing member is constructed to deform with and to accommodate to the decreasing radial spacing of the peripheral gap as the electrode heats up due to arc lamp operation.

4. In an arc lamp in accordance with claim 1 in which each packing member comprises a coil of refractory metal wire wound around the electrode portion.

5. In an arc lamp in accordance with claim 4 in which said coil of wire is of helical configuration and is tightly sprung upon the electrode portion.

6. In an arc lamp in accordance with claim 4 in which said coil of wire is secured to said electrode portion at no more than one point thereby allowing the wire of the coil at either side of said point of securement to wind and unwind relative to said electrode portion within said gap.

7. In an arc lamp in accordance with claim 6 in which one end of said coil of wire is secured to the electrode portion.

8. In an arc lamp in accordance with claim 6 in which the envelope portions coextensive with said coils of wire are spirally grooved to engage said coil of wire longitudinally and to thereby support said electrodes securely against longitudinal displacement Within said envelope.

9. In an arc lamp in accordance with claim 1 in which said packing member comprises a flat spiral spring of refractory material.

10. In an arc lamp in accordance with claim 9 in which an end portion of said ilat spiral spring is secured to said electrode portions said grooves in said envelope engaging at least one of the edges of said spiral spring to support said electrode against longitudinal displacement within said envelope.

11. In an arc lamp in accordance with claim 1 in which said packing member comprises a bellows-type metallic sleeve having circular ridges.

12. -In an arc lamp in accordance with claim 11 in which a portion of said sleeve is secured to an electrode portion, said grooves in said envelope engaging at least one edge of said sleeve to support said electrode against longitudinal displacement within said envelope.

13. In an arc lamp in accordance with claim 1 in which said packing member comprises a lengthwise corrugated metallic sleeve having linear ridges.

14. An arc lamp comprising:

a sealed envelope;

a pair of electrodes within said sealed envelope having a foot portion, a body portion, and an arc end portion, said envelope supportingly engaging the foot portions of said electrodes to thereby support said electrodes in cantilever fashion in spaced apart relationship to define an arc gap therebetween;

a packing member mounted about said body portions and engageable with said envelope for supporting said electrodes against both longitudinal and lateral displacement, said envelope being shrunk about said packing member so as to form a groove in said envelope for receiving said packing member; and

said packing member being elastically deformable within said groove by the forces generated as the result of substantially different coefficients of linear expansion of said envelope and said electrodes to support said electrodes both longitudinally and laterally at all electrodes temperatures.

15. An arc lamp in accordance with claim 14 in which said packing member comprises a helical coil of refractory wire wound around said body portion and attached to said body portion at no more than one point to freely allow said coil of wire to wind and unwind within said groove and about said body portion.

References Cited UNITED STATES PATENTS 1,858,737 5/1932 Hendry 313-284 X 1,972,611 9/1934 Warren 313-288 2,443,618 6/ 1948 Homan 313-288 2,716,584 8/ 1955 Retzer 316-19 RAYMOND F. HOSSFELD, Examiner U.S. Cl. X.R.

PAM-22 NO- z y :1s Yum Dated Imp m T 1 Q/n Vnwr) Raymond E Paquet te It is certified that error appears in the above-identified patent and that said Letters Patent are nefeby corrected as shown below:

Column l, in the heading; change the assignee designation from "to Gevaert-Agfa N.V. Mortsel,

Belgium, a Belgian Company" to by mesne assignments, to Varo, Inc. Garland, Texas, a

Corporation of Texas.

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